This application claims the priority of Korean Patent Application No. 10-2003-0083618, filed on Nov. 24, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical communication system, and more particularly, to a feedback amplifier used as a preamplifier of an optical communication system.
2. Description of the Related Art
A general feedback amplifier includes an amplifier circuit for amplifying an input signal in order to provide an output voltage. In such a feedback amplifier, a feedback circuit for controlling levels of an output voltage signal is provided. The feedback circuit includes a feedback resistor connected between an input terminal and an output terminal and a transistor connected in parallel to the feedback resistor. The transistor receives a control signal through its base. Also, feedback current flowing through the feed circuit is controlled according to the control signal, thereby controlling an output voltage.
Such a feedback amplifier is disclosed in detail in Japanese Patent Publication No. 57-194613. The feedback amplifier will be schematically described below.
FIG. 1 is a circuit diagram of a general feedback amplifier 10 disclosed in the Japanese Patent Publication No. 57-194613. Referring to FIG. 1, the feedback amplifier 10 is connected between an input terminal IN and an output terminal OUT. The input terminal IN is connected to a photodiode PD connected with a power supply VCC. Input current in corresponding to cathode current of the photodiode PD is applied to the input terminal IN.
The input terminal IN is connected to the base of a NPN transistor 11. The collector of the NPN transistor 11 is connected to the power supply VCC and the base of a NPN transistor 13 through a resistor 12. The collector of the NPN transistor 13 is connected to the power supply VCC and the emitter thereof is connected to the output terminal OUT and a ground through a resistor 14. Also, the emitter of the NPN transistor 11 is grounded.
The output terminal OUT is connected to the base of the transistor 11 through a resistor 15a located in a feedback circuit 15. The emitter of a transistor 15b located in the feedback circuit 15 is connected to the output terminal OUT and the collector of the transistor 15b is connected to the base of the transistor 11. A control signal V_AGC for controlling the feedback current is applied from an external circuit (not shown) to the base of the transistor 15b. 
In such a feedback amplifier, if a control signal V_AGC of a low voltage is applied to the base of the transistor 15b, the transistor 15b is turned off so that trans-impedance between its collector and emitter sufficiently increases. Accordingly, the transistor 15b is open and the trans-impedance has the same resistance as a resistor 15a. 
Thereafter, if the voltage of the control signal V_AGC increases, the transistor 15b is turned on and accordingly trans-impedance between its collector and emitter decreases. As a result, feedback resistance is reduced and trans-impedance is also reduced. As such, the gain of the feedback amplifier 10 is controlled according to the control signal V_AGC.
In other words, in the feedback amplifier 10, if a control signal V_AGC changes according to input current in, the control signal V_AGC should be set based on the input current until an output voltage S10 is saturated. Also, after a separate gain control signal generation circuit is formed in order to provide an appropriate signal value, the control signal V_AGC is applied to the feedback amplifier 10 through the gain control signal generation circuit.
However, due to such a separate gain control signal generation circuit required to obtain a wide dynamic range, an entire circuit configuration is complicated and controlling of the dynamic range is difficult.